1. Field of the Invention
The present invention relates to motors, and more particularly to a synchronous motor with reduced noise and vibrations in a combination of rotor structures.
2. Description of the Related Art
Japanese Patent Laid-Open Publication No. Hei 9-294362 discloses a Conventional synchronous motor that has different types of rotors axially coupled to each other for providing an enhanced flexibility in design. Referring to FIG. 12, a motor 1 comprises a rotor 2 having a permanent magnet rotor unit 4 and a reluctance rotor unit 6 axially coupled to each other, and a stator 3 for generating a field for driving the rotor 2. In this arrangement, the permanent magnet rotor unit 4 has a plurality of permanent magnets disposed therein, and the reluctance rotor unit 6 has a plurality of salient pole portions formed therein.
Japanese Patent Laid-Open Publication No. Hei 7-59310 discloses another conventional motor, in which different types of rotors are axially coupled to each other with a nonmagnetic material interposed therebetween for providing an enhanced flexibility in design in the same manner, and preventing flux leakage and thereby magnetic coupling between the different types of rotors. The motor is also configured to shift the different types of rotors, when combined axially, by a predetermined angle deviation in the direction of rotation and thereby provide reduced torque pulsation.
However, the conventional motor disclosed in Japanese Patent Laid-Open Publication No. Hei 9-294362 having different types of rotors being combined to each other, cannot prevent the motor from being deterioration in characteristics due to the interaction between the rotors. In other words, the structure that employs one rotor having permanent magnets covered with the core of the other rotor cannot prevent the flux leakage of the permanent magnet to the other rotor, thereby causing deterioration in characteristics.
On the other hand, the conventional motor disclosed in Japanese Patent Laid-Open Publication No. Hei 7-59310 has the different types of rotors axially coupled to each other with the nonmagnetic material therebetween, and the nonmagnetic material prevents deterioration in characteristics caused by flux leakage. However, arrangement of the nonmagnetic material increases size and costs of the motor. Torque pulsation of the motor is reduced by axially combining the different types of rotors with a predetermined angle deviation in the direction of rotation. However, cogging torque generated from the permanent magnet causes the motor to produce torque pulsation, resulting in generating of noise and vibrations of the motor. Accordingly, cogging torque generated by at least one of the different types of rotors is higher, reduction in noise and vibration of the motor may be insufficient.